Hi, Your choice may be subject to the speakers you intend to use with either of these amplifiers. Keep in mind that the ST-35 is essentially half (17.5wpc) of the ST-70 (35wpc). Accordingly, I would suggest matching the ST-35 with a fairly efficient (90+ db) pair of speakers whereas with the ST-70 (80+ db)should be fine. There is no need to crank up the volume to ear bleeding decibels to optimize the sound of either these tube types.

I agree with Kevin. The ST-35 is $125 to $150 less expensive but has only half the power of the ST-70. The ST-35 has a few other disadvantages.

1. The ST-35 has no power switch so unless you want to add a power switch or plug the amp into a switched outlet you will wind up having to plug in and unplug the amp every time you want it on or off.

2. The ST-35 can only use EL84 output tubes while a modern ST-70 kit can use EL34, 6L6, KT66, KT77, KT88 or 6550 output tubes. Each tube has a slightly different sonic signature and you can tailor the sound to your liking by swapping out different types of output tubes.

3. In stock form the ST-35 has no adjustable bias system. You really need a matched quad of EL84's in an ST-35. As the tubes wear there is no way to compensate for tube wear by readjusting the bias like on an ST-70. That said, Dynakitparts does have and adjustable bias system for the ST-35 that you can add to any ST-35 to alleviate this issue. See link below.